Two-electrode touch button with a multi-phase capacitance measurement process

Abstract

A method, apparatus, and system to detect whether a two-electrode touch button is pressed using a first self-capacitance measurement of an inner electrode of the two-electrode touch button and a second self-capacitance measurement of an outer electrode of the two-electrode touch button. The method, apparatus, and system further to detect whether the two-electrode touch button is pressed in view of presence of water proximate to the touch button.

What is claimed is: 
     
       1. A device comprising:
 a capacitance measurement circuit to couple to a touch button comprising an inner electrode substantially surrounded by an outer electrode; and 
 a processing element to couple to the capacitance measurement circuit, wherein the processing element is to determine whether the touch button is pressed using a two-stage measurement process comprising:
 a first stage in which a first self-capacitance measurement of the inner electrode is made by the capacitance measurement circuit while an active shield signal is coupled to the outer electrode; and 
 a second stage in which a second self-capacitance measurement of the outer electrode is made while a fixed voltage is coupled to the inner electrode. 
 
 
     
     
       2. The device of  claim 1 , wherein the capacitance measurement circuit comprises a ratiometric capacitance measurement circuit. 
     
     
       3. The device of  claim 1 , wherein the capacitance measurement circuit comprises a multi-configuration self-capacitance measurement circuit. 
     
     
       4. The device of  claim 1 , wherein the inner electrode comprises a first surface area and the outer electrode comprises a second surface area, wherein the first surface area and the second surface area are substantially similar. 
     
     
       5. The device of  claim 1 , wherein the capacitance measurement circuit comprises:
 a selection circuit; and 
 a capacitance-to-digital conversion circuit coupled to the selection circuit, and wherein the processing element comprises water rejection logic to determine whether the touch button is pressed using the first self-capacitance measurement and the second self-capacitance measurement. 
 
     
     
       6. The device of  claim 5 , wherein the capacitance-to-digital conversion circuit comprises:
 a programmable current source selectively coupled to a first node; 
 a driver comprising an input coupled to a voltage reference and an output selectively coupled to a second node; 
 a comparator coupled to the first node and the voltage reference; 
 a flip-flop coupled to an output of the comparator; 
 a decimator coupled to an output of the flip-flop and selectively coupled to the first node; and 
 a modulator capacitor coupled between the first node and ground. 
 
     
     
       7. The device of  claim 1 , further comprising:
 a first multiplexer coupled to the inner electrode, the first multiplexer to selectively couple the inner electrode to the capacitance measurement circuit in the first stage and selectively couple the inner electrode to a fixed voltage node in the second stage; and 
 a second multiplexer coupled to the outer electrode, the second multiplexer to selectively couple the outer electrode to a voltage reference in the first stage and selectively couple the outer electrode to the capacitance measurement circuit in the second stage. 
 
     
     
       8. The device of  claim 7 , wherein the fixed voltage node is a ground terminal. 
     
     
       9. The device of  claim 7 , wherein the capacitance measurement circuit further comprises:
 a first switch coupled between a first node and a third node of the capacitance measurement circuit, the first switch to connect the first node and the third node in a first phase of the capacitance measurement circuit; 
 a second switch coupled between the third node and ground, the second switch to ground the third node in a second phase of the capacitance measurement circuit; 
 a third switch coupled between the driver and a second node of the capacitance measurement circuit, the third switch to drive the voltage reference on the second node in the first phase; and 
 a fourth switch coupled between the second node and ground, the fourth switch to ground the second node in the second phase. 
 
     
     
       10. A method comprising:
 coupling an inner electrode of a two-electrode touch button to a capacitance measurement circuit in a first stage of a two-stage measurement process; 
 coupling an outer electrode of the two-electrode button to an active shield signal in the first stage, the inner electrode being substantially surrounded by the outer electrode; 
 measuring, by the capacitance measurement circuit, a first self-capacitance measurement of the inner electrode while the active shield is coupled to the outer electrode in the first stage; 
 coupling the outer electrode to the capacitance measurement circuit in a second stage of the two-stage measurement process; 
 coupling the inner electrode to a fixed voltage in the second stage; 
 measuring, by the capacitance measurement circuit, a second self-capacitance measurement of the outer electrode while the fixed voltage is coupled to the inner electrode; and 
 determining whether the two-electrode touch button is pressed using the first self-capacitance measurement and the second self-capacitance measurement. 
 
     
     
       11. The method of  claim 10 , further comprising:
 comparing the first self-capacitance measurement against a first enable threshold; 
 comparing the second self-capacitance measurement against a second enable threshold; 
 comparing a difference value between the first self-capacitance measurement and the second self-capacitance measurement against a third enable threshold; 
 detecting activation of the two-electrode touch button as being pressed when 1) the first self-capacitance measurement is greater than the first enable threshold, 2) the second self-capacitance measurement is greater than the second enable threshold, and 3) the difference value is greater than the third enable threshold; and 
 detecting deactivation of the two-electrode touch button as not being pressed when any one of 1), 2), or 3) is false. 
 
     
     
       12. The method of  claim 11 , further comprising:
 resetting stored attributes of the two-electrode touch button when 1) a water flag is set; 
 resetting the stored attributes when 2) the first self-capacitance measurement is not greater than a touch noise threshold and 3) the second self-capacitance measurement is greater than the second enable threshold; 
 resetting the stored attributes when 4) the water flag is set and 5) the second self-capacitance measurement is less than half a water enable threshold; 
 setting the water flag when 6) the second self-capacitance measurement is greater than the water enable threshold and 7) the first self-capacitance measurement is greater than the second enable threshold; and 
 detecting deactivation of the two-electrode touch button as not being pressed when 7) is false. 
 
     
     
       13. The method of  claim 10 , further comprising:
 comparing the first self-capacitance measurement against a first enable threshold; 
 comparing the second self-capacitance measurement against a second enable threshold; 
 comparing a ratio of the first self-capacitance measurement and the second self-capacitance measurement against a third enable threshold; 
 detects activation of the two-electrode touch button as being pressed when 1) the first self-capacitance measurement is greater than the first enable threshold, 2) the second self-capacitance measurement is greater than the second enable threshold, and 3) the ratio is greater than the third enable threshold; and 
 detects deactivation of the two-electrode touch button as not being pressed when any one of 1), 2), or 3) is false. 
 
     
     
       14. The method of  claim 11 , further comprising:
 resetting stored attributes of the two-electrode touch button when 1) a water flag is set; 
 resetting the stored attributes when 2) the first self-capacitance measurement is not greater than a touch noise threshold and 3) the second self-capacitance measurement is greater than the second enable threshold; 
 resetting the stored attributes when 4) the water flag is set and 5) the second self-capacitance measurement is less than half a water enable threshold; 
 setting the water flag when 6) the second self-capacitance measurement is greater than the water enable threshold and 7) the first self-capacitance measurement is greater than the second enable threshold; and 
 detects deactivation of the two-electrode touch button as not being pressed when 7) is false. 
 
     
     
       15. The method of  claim 10 , further comprising:
 comparing the first self-capacitance measurement against a first enable threshold; 
 comparing the second self-capacitance measurement against a second enable threshold; 
 comparing a difference value between the first self-capacitance measurement and the second self-capacitance measurement against a third enable threshold; 
 comparing a ratio of the first self-capacitance measurement and the second self-capacitance measurement against a fourth enable threshold; 
 detects activation of the two-electrode touch button as being pressed when 1) the first self-capacitance measurement is greater than the first enable threshold, 2) the second self-capacitance measurement is greater than the second enable threshold, 3) the difference value is greater than the third enable threshold, and 4) the ratio is greater than the third enable threshold; and 
 detects deactivation of the two-electrode touch button as not being pressed when any one of 1), 2), 3), or 4) is false. 
 
     
     
       16. A system comprising:
 a two-electrode touch button comprising an inner electrode substantially surrounded by an outer electrode; and 
 a processing device coupled to the two-electrode touch button, wherein the processing device comprises:
 a capacitance measurement circuit selectively coupled to the inner electrode and the outer electrode of the two-electrode touch button; and 
 a processing element coupled to the capacitance measurement circuit, wherein the processing element is to determine whether the two-electrode touch button is pressed using a two-stage measurement process comprising:
 a first stage in which a first self-capacitance measurement of the inner electrode is made by the capacitance measurement circuit while an active shield signal is coupled to the outer electrode; and 
 a second stage in which a second self-capacitance measurement of the outer electrode is made while a fixed voltage is coupled to the inner electrode. 
 
 
 
     
     
       17. The system of  claim 16 , wherein the capacitance measurement circuit comprises at least one of a ratiometric capacitance measurement circuit or a multi-stage self-capacitance measurement circuit. 
     
     
       18. The system of  claim 16 , wherein the capacitance measurement circuit comprises:
 a selection circuit; and 
 a capacitance-to-digital conversion circuit coupled to the selection circuit, and wherein the processing element comprises water rejection logic to determine whether the touch button is pressed using the first self-capacitance measurement and the second self-capacitance measurement. 
 
     
     
       19. The system of  claim 18 , wherein the capacitance-to-digital conversion circuit comprises:
 a programmable current source selectively coupled to a first node; 
 a driver comprising an input coupled to a voltage reference and an output selectively coupled to a second node; 
 a comparator coupled to the first node and the voltage reference; 
 a flip-flop coupled to an output of the comparator; 
 a decimator coupled to an output of the flip-flop and selectively coupled to the first node; and 
 a modulator capacitor coupled between the first node and ground. 
 
     
     
       20. The system of  claim 16 , wherein the processing device further comprises:
 a first multiplexer coupled to the inner electrode, the first multiplexer to selectively couple the inner electrode to the capacitance measurement circuit in the first stage and selectively couple the inner electrode to a fixed voltage node in the second stage; and 
 a second multiplexer coupled to the outer electrode, the second multiplexer to selectively couple the outer electrode to a voltage reference in the first stage and selectively couple the outer electrode to the capacitance measurement circuit in the second stage.